Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. Each of the data tracks extends generally circumferentially around the disk and can store data in the form of magnetic transitions within the radial extent of the track on the disk surface. Typically, each data track is divided into a number of data sectors that store fixed sized data blocks.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to transmit an electrical signal that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a read/write gap that positions the active elements of the transducer at a position suitable for interaction with the magnetic transitions on the surface of the disk, as the disk rotates.
As known in the art, the magnetic transducer is mounted by the head to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disk to either read data from or write data to the preselected data track, as the disk rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disk surface due to fluid currents caused by rotation of the disk.
An important aspect of conventional disk drive design concerns position control of the head. A position control system is used to accurately position a head over a data track during data read and write operations. Whenever data are either written to or read from a particular data track, the transducer gap of the head should be centered over the centerline of the magnetic transitions of the data track where the data are to be written or from where the data are to be read, to assure accurate transduction of the transitions representing data. If the head is off-center, the head may transduce (i.e. either read or write, as the case may be) transitions from an adjacent track, and thereby corrupt the data.
A closed loop servo system is typically used to control the position of the actuator arm. In a known servo system, servo position information is recorded on the disk surface itself, and periodically read by the head for use in controlling the position of the actuator arm. Such a servo arrangement is referred to as an embedded servo system. In modern disk drive architectures utilizing an embedded servo system, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector, as noted above. In addition, associated with the data sectors are a series of servo sectors that are generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors, as is well known.
Typically each servo sector is radially aligned with corresponding servo sectors of neighboring data tracks to form a set of radially extending, spoke-like servo sections that are equally spaced from one another around the circumference of the disk surface. The equal spacing between servo sectors provides a fixed frequency of servo occurrences regardless of the radial position of the head. However, when data are recorded in a zone bit arrangement, the number of data sectors within one rotation of a disk varies from zone-to-zone, thus causing the precise locations of servo sectors of the spoke-like sections, relative to the data fields of the data sectors, to vary from zone-to-zone and within a zone.
A zone bit arrangement is a known technique to maximize the storage capacity of a disk. In accordance with the fundamental geometry of a circle, the circumferences of the data tracks increase in a direction toward the outer diameter of the disk. Thus, each succeeding data track in the radially outward direction, has more potential data storage capacity than the preceding data tracks. A zone bit recording scheme takes advantage of the increasing circumference aspect of circle geometry. In a zone bit recording, the surface of the disk is divided into a set of zones. Each zone extends for a fixed radial length, and the magnetic transition frequency is increased from zone-to-zone, in the radially outward direction. Accordingly, the number of data sectors in each track increases, from zone-to-zone, in the radially outward direction.
In an embedded servo system, each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine bead position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a microprocessor within the disk drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
In a conventional disk drive design, each data field of a sector includes an address header comprising magnetic transitions that represent unique identification information for the specific data stored in the data sector. In this manner, the disk drive system can locate and verify the exact data sector for any particular block of data that a host computer may require, e.g., in a read operation. Among the information stored in an address header of a zone bit recording format is a split count to indicate the number of data bytes in the data field until a servo sector splits the data field. The electronics system in the disk drive uses the split count information to suspend data read or write processing while the head is over the servo sector. The control is typically handled directly by a disk controller circuit that receives the header split count information.
A disk controller circuit is used in conventional disk drive designs to receive, during a read operation, serial signals derived from the transduction of magnetic transitions that represent bits of information. The disk controller operates to de-serialize the bits into eight bit bytes for transfer to the host computer. An eight bit byte is the data structure typically used by a host computer when processing, storing or transmitting data. The disk controller also operates to serialize bytes received from the host computer into a stream of bits, during a write operation, for transfer to a disk surface. In this manner, the disk controller permits the transfer of data between the byte architecture of a host computer and the serial storage architecture of a disk drive.
Overhead refers to portions of a disk surface that are used to store information necessary for the control of the disk drive. Space on a disk surface used to store control information is not available to store data, and thus reduces the storage capacity of the disk drive. The servo sectors and address headers discussed above are examples of overhead. A split count can occupy two bytes of a header and thus contributes to overhead.
Another system for handling split data fields comprises a register that stores servo sector locations for each of the zones of the disk surface. This approach removes the need for incurring overhead on the disk surface for the storage of split count information. During read or write operation of the disk drive, a current servo sector location is output from the register, and an appropriate data clock increments a counter. An output of each of the counter and register is coupled to a comparator that operates, during each clock cycle, to compare the location register output to the current counter output. When these values are equal, the comparator transmits a signal to a control unit, that, in turn, controls switches to "freeze" the operation of the disk controller circuit by opening electric paths between the disk controller circuit and clock signals used to control circuit operation.
The known location register approach, however, makes the servo split control transparent to the disk controller, and thus removes the ability to directly and precisely control disk controller operations in connection with the handling of data read and write operations in the presence of servo sectors that split data fields. For example, disk drives typically control read and write operations via the assertion of READ GATE (RG) and WRITE GATE (WG) signals. The disk controller is responsive to the RG and WG signals to control operations that define byte boundaries when either serializing or de-serializing bits. The known location register approach is not cognizant of the state of the RG and WG signals, but simply freezes disk controller operation, from a control point that is external to the disk controller, at a time that should coincide with the arrival of the head over a servo sector. Thus, while the known location register approach reduces overhead on the disk surfaces, there is no assurance that disk controller operations are frozen exactly at a byte boundary between the last byte before the split and the servo sector.